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| United States Patent |
6,251,216
|
|
Okamura
, et al.
|
June 26, 2001
|
Apparatus and method for plasma processing
Abstract
A plasma processing apparatus includes a reaction chamber in which plasma
is generated from a reactive gas introduced thereto and a film on a
substrate is processed with the plasma generated. The main members of the
reaction chamber are covered with protective members made of synthetic
quartz.
| Inventors:
|
Okamura; Hideaki (Osaka, JP);
Imai; Shinichi (Osaka, JP);
Jiwari; Nobuhiro (Osaka, JP);
Tohmori; Yoko (Osaka, JP)
|
| Assignee:
|
Matsushita Electronics Corporation (Osaka, JP)
|
| Appl. No.:
|
212411 |
| Filed:
|
December 16, 1998 |
Foreign Application Priority Data
| Current U.S. Class: |
156/345.1; 118/723E; 118/728; 156/916; 216/67; 216/71; 438/710; 438/723; 438/729 |
| Intern'l Class: |
H05H 001/00 |
| Field of Search: |
156/345
118/723 E,723 R,728
216/67,71
438/710,729,723
|
References Cited
U.S. Patent Documents
| 5435880 | Jul., 1995 | Minato et al. | 156/345.
|
Primary Examiner: Dang; Thi
Attorney, Agent or Firm: Robinson; Eric J.
Nixon Peabody LLP
Claims
What is claimed is:
1. A plasma processing apparatus comprising a reaction chamber in which
plasma is generated from a reactive gas introduced thereto and a film on a
substrate is processed with the plasma generated,
wherein main members of the reaction chamber are covered with protective
members made of synthetic quartz,
wherein an impurity is contained in the synthetic quartz at less than 0.1
ppm.
2. A plasma processing apparatus comprising a reaction chamber in which
plasma is generated from a reactive gas introduced thereto and a film on a
substrate is processed with the plasma generated,
wherein main members of the reaction chamber are covered with protective
members made of synthetic quartz,
wherein an impurity contained in the synthetic quartz is selected from the
group consisting of sodium, calcium, potassium, iron and magnesium.
3. A plasma processing apparatus comprising a reaction chamber in which
plasma is generated from a reactive gas introduced thereto and a film on a
substrate placed on a sample stage is processed with the plasma generated,
wherein a ring member made of synthetic quartz is disposed around a region
of the sample stage where the substrate is placed.
4. The apparatus of claim 3, wherein an impurity is contained in the
synthetic quartz at less than 0.1 ppm.
5. The apparatus of claim 3, wherein an impurity contained in the synthetic
quartz is selected from the group consisting of sodium, calcium,
potassium, aluminum, iron and magnesium.
6. A plasma processing method comprising the steps of:
placing a substrate, on the surface of which a silicon dioxide film has
been formed, on a substrate placing region of a sample stage provided in a
reaction chamber, and disposing a ring member made of synthetic quartz
around the substrate placing region of the sample stage;
introducing a reactive gas into the reaction chamber and generating plasma
from the reactive gas introduced; and
etching the silicon dioxide film with the plasma.
Description
BACKGROUND OF THE INVENTION
The present invention generally relates to apparatus and method for plasma
processing. Specifically, the present invention relates to a plasma
processing apparatus including a reaction chamber where plasma is
generated from a reactive gas introduced thereto and a film on a substrate
is processed with the plasma generated, and to a plasma processing method
for performing plasma etching in the reaction chamber.
Plasma processing methods are applied to various kinds of processing steps
like etching a film on a substrate with plasma to form a fine-line
pattern, and play a significant role in the fabrication process of a
semiconductor device.
The main members (e.g., the walls) of a reaction chamber for performing
various types of plasma processing such as plasma etching are ordinarily
made of aluminum. However, aluminum acts as an acceptor (i.e., p-type
impurity) against silicon, which is a typical material of a semiconductor
substrate. Accordingly, it is necessary to prevent aluminum, coming from
an etched part of a main member of the reaction chamber such as a wall
thereof, from diffusing on the semiconductor substrate. Thus, the main
members of the reaction chamber are usually covered with protective
members lest the main members should be etched by plasma.
A protective member is ordinarily made of alumina (aluminum oxide) obtained
by subjecting the surface of aluminum to an anodic oxide coating treatment
(i.e., a so-called "ALUMITE " treatment), because this treatment is easy.
In general, alumina is hard to be etched. Accordingly, if a protective
member, made of aluminum including alumina regions on the surface thereof,
is used, then impurities generated from the protective member can be
reduced.
Recently, such protective members, covering the main members of a reaction
chamber, not only plays the originally intended role of protecting the
main members from plasma, but also is positively used for controlling the
etching characteristics. Its objective is to properly deal with the
everlasting miniaturization of semiconductor integrated circuits. Also,
since the density of plasma has been drastically increased nowadays, the
state of plasma affects the etching characteristics to a far larger degree
than what it used to be. In view of these situations, the state of plasma
is desirably kept as uniform and constant as possible. For example,
suppose a silicon dioxide film on a semiconductor substrate is etched with
plasma. In such a case, if the main members of a reaction chamber are
covered with protective members made of quartz, which has the same
composition as the silicon dioxide film, then a region where plasma has
been generated is surrounded by silicon dioxide (or quartz). Accordingly,
the plasma generated in the region equally etches the silicon dioxide film
on the semiconductor substrate and the protective members of the reaction
chamber under the same conditions. As a result, the state of the plasma is
invariable with the passage of time and spatially uniform.
Nevertheless, if the main members of a reaction chamber are covered with
such protective members made of quartz, then the quartz protective members
are also etched unintentionally as the etching on the silicon dioxide film
proceeds. However, the substance resulting from the etched quartz of the
protective member acts as neither acceptor nor donor against silicon
unlike aluminum. Accordingly, even if such a substance diffuses onto the
semiconductor substrate, the electrical characteristics of a semiconductor
device are not adversely affected by the substance.
Because of these reasons, quartz is superior to aluminum as a material for
the protective members of the main members of a reaction chamber used for
performing plasma etching on a semiconductor integrated circuit component,
which should be formed in an even smaller size.
Also, according to a proposed technique, alumina and quartz are used in
combination. Specifically, alumina is used for protective members covering
the main members of a reaction chamber, because alumina has the
above-described advantages. And a ring member made of quartz is disposed
around a substrate placed on a sample stage, because the etching
characteristics are particularly affected by the state of plasma on the
periphery of a film to be processed. The objective thereof will be
described below. If a silicon dioxide film, or the film to be processed,
and the member disposed around the substrate on the sample stage are made
of different materials, then the objects in contact with the plasma are
greatly different from each other in the vicinity of the interface between
the periphery of the silicon dioxide film and the sample stage.
Accordingly, the state of the plasma on the periphery of the silicon
dioxide film is different from the state of the plasma at the center of
the silicon dioxide film. As a result, the characteristics of etching on
the silicon dioxide film are variable within the plane of the silicon
dioxide film. Therefore, in order to keep the state of plasma constant,
the quartz ring member is disposed around the substrate placed on the
sample stage.
If the main members of a reaction chamber are covered with quartz
protective members or if a quartz ring member is disposed around a
substrate placed on a sample stage, then the state of plasma can be surely
kept constant and uniform. However, even in such a case, it is still
impossible to prevent impurities, deposited on a film to be processed,
from adversely affecting the electrical characteristics of a semiconductor
device. This problem will be briefly discussed below.
For example, if impurities are deposited on a film to be processed, then a
pause time (i.e., a charge retention time per memory cell), one of the
parameters for evaluating the reliability of a DRAM, disadvantageously
decreases. It is considered that the pause time probably decreases because
of the generation of leakage current resulting from the deposition of
impurities such as sodium onto a memory cell of the DRAM.
Thus, it has been believed an urgent task to prevent impurities, contained
in reactants flaked off the main or protective members of a reaction
chamber, from being deposited on a film to be processed by regularly
cleaning the main and protective members and removing the deposited
reactants therefrom.
In order to confirm the effects of such regular cleaning, the present
inventors carried out experimental plasma etching with a reduced amount of
reactants deposited on the main and protective members of the reaction
chamber by cleaning these members regularly and carefully. However, we
faced a problem that the pause time of a DRAM still could not be
increased.
SUMMARY OF THE INVENTION
In view of these problems, the objects of this invention is to keep plasma
as uniform as possible by covering the main members of a reaction chamber
with quartz protective members and/or disposing a quartz ring member
around a substrate placed on a sample stage and to reduce impurities
deposited on a film placed on the substrate.
In order to accomplish these objects, the present inventors analyzed from
various angles the reasons why the impurities, deposited on the film to be
processed, are generated. As a result, we arrived at the following
conclusions.
The first conclusion is: the pause time of a DRAM obtained by plasma
etching with the main members of a reaction chamber covered with quartz
protective members is inferior to that of a DRAM obtained by plasma
etching with the main members covered with alumina protective members.
Based on this result, we performed experiments on the pause time of a DRAM
obtained by plasma etching with the main members of a reaction chamber
covered with alumina protective members. Consequently, we reached the
second conclusion that the pause time of a DRAM obtained with a quartz
ring member disposed around a substrate placed on a sample stage is
inferior to that of a DRAM obtained with the ring member not disposed
there.
Thus, we suspected that the quartz protective member or ring member might
be the source of generating impurities, and analyzed the material quality
of quartz as the component of the protective and ring members and the
amounts of impurities contained in quartz. As a result, we found the
following facts. First, quartz used for the protective or ring members is
fused quartz. Second, the amounts of the impurities contained in fused
quartz are large enough to affect the recent dramatic increase in density
of plasma and drastic downsizing of a semiconductor integrated circuit.
And, the amounts of impurities contained in synthetic quartz are
considerably smaller than those of impurities contained in fused quartz.
The present invention was conceived based on these findings, and is
characterized by using synthetic quartz, instead of fused quartz
conventionally used, as a material for protective members covering the
main members of a reaction chamber or a ring member disposed around a
substrate.
Specifically, a first plasma processing apparatus according to the present
invention includes a reaction chamber in which plasma is generated from a
reactive gas introduced thereto and a film on a substrate is processed
with the plasma generated. The main members of the reaction chamber are
covered with protective members made of synthetic quartz.
In the first plasma processing apparatus, the main members of a reaction
chamber are covered with protective members made of synthetic quartz.
Thus, when a silicon dioxide film is etched with plasma in such a reaction
chamber, the silicon dioxide film and the protective members are both
etched by the plasma on the same conditions. Accordingly, the plasma is
kept in a uniform state both temporally and spatially. In this case, the
plasma generated in the reaction chamber etches the protective members.
But since the protective members made of synthetic quartz contain very
small amounts of impurities, the amounts of impurities released from the
protective members or deposited on the surface of a film to be processed
are extremely small. As a result, the pause time of a DRAM formed on the
substrate can be increased.
A second plasma processing apparatus according to the present invention
includes a reaction chamber in which plasma is generated from a reactive
gas introduced thereto and a film on a substrate placed on a sample stage
is processed with the plasma generated. A ring member made of synthetic
quartz is disposed around a region of the sample stage where the substrate
is placed.
In the second plasma processing apparatus, a ring member made of synthetic
quartz is disposed around a region of the sample stage where the substrate
is placed. Thus, when a silicon dioxide film on the substrate is etched
with plasma, the silicon dioxide film, which is the film to be processed,
is made of the same material as that of the surrounding member around the
film. Accordingly, since the plasma on the periphery of the film to be
processed is in the same state as the plasma at the center of the film,
the characteristics of etching on the film to be processed is uniform over
the entire plane of the film. In this case, the plasma generated in the
reaction chamber etches the ring member. But since the ring member made of
synthetic quartz contains very small amounts of impurities, the amounts of
impurities released from the ring member or deposited on the surface of a
film to be processed are extremely small. As a result, the pause time of a
DRAM formed on the substrate can be increased.
In the first or second plasma processing apparatus, an impurity is
contained in the synthetic quartz at preferably less than 0.1 ppm.
In such an embodiment, even though plasma etches the protective members or
the ring member, substantially no impurities are released from the
protective members or the ring member. Accordingly, the pause time of a
DRAM formed on the substrate can be even more increased.
In this case, an impurity contained in the synthetic quartz may be selected
from the group consisting of sodium, calcium, potassium, aluminum, iron
and magnesium. If the impurity is any of these materials, the adverse
effects of the impurities on the pause time of a DRAM can be eliminated
with a lot more certainty.
A plasma processing method according to the present invention includes the
steps of: placing a substrate, on the surface of which a silicon dioxide
film has been formed, on a substrate placing region of a sample stage
provided in a reaction chamber, and disposing a ring member made of
synthetic quartz around the substrate placing region of the sample stage;
introducing a reactive gas into the reaction chamber and generating plasma
from the reactive gas introduced; and etching the silicon dioxide film
with the plasma.
In accordance with the plasma processing method of the present invention, a
ring member made of synthetic quartz is disposed around a region of the
sample stage where the substrate is placed. Thus, a silicon dioxide film,
which is the film to be processed, is made of the same material as that of
the surrounding member around the film. Accordingly, since the plasma on
the periphery of the film to be processed is in the same state as the
plasma at the center of the film, the characteristics of etching on the
film to be processed is uniform over the entire plane of the film. Also,
since the ring member made of synthetic quartz contains very small amounts
of impurities, the amounts of impurities released from the ring member or
deposited on the surface of a film to be processed are extremely small
even though the plasma etches the ring member. As a result, the pause time
of a DRAM formed on the substrate can be increased.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of a plasma processing apparatus according
to the first embodiment of the present invention.
FIG. 2 is a cross-sectional view illustrating the principal portion,
corresponding to the portion A encircled by the one dot chain in FIG. 1,
of a conventional plasma processing apparatus.
FIG. 3 is a graph illustrating the relationship between the content of
impurities in protective members and the amount of impurities deposited on
the surface of a substrate.
FIG. 4 is a graph illustrating the relationship between the amount of
impurities deposited on the surface of a substrate and a pause time (i.e.,
charge retention time).
FIG. 5 is a cross-sectional view illustrating the principal portion of a
plasma processing apparatus according to the second embodiment of the
present invention.
FIGS. 6(a) and 6(b) are respectively plan view and cross-sectional view
illustrating in detail a ring member for the plasma processing apparatus
of the second embodiment.
FIGS. 7(a) through 7(d) are cross-sectional views illustrating respective
process steps of plasma etching performed using the plasma processing
apparatus of the second embodiment.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
EMBODIMENT 1
Hereinafter, a plasma processing apparatus according to the first
embodiment of the present invention will be described with reference to
FIGS. 1 through 4.
FIG. 1 illustrates the cross-sectional structure of the plasma processing
apparatus of the first embodiment. As shown in FIG. 1, a sample stage 11
is provided as a lower electrode inside a reaction chamber 10, and has a
substrate holding portion 11a for holding a semiconductor substrate 12
thereon. An upper electrode 13 is provided inside the reaction chamber 10
so as to face the substrate holding portion 11a of the sample stage 11. A
large number of gas dispersing holes 13a are formed in the upper electrode
13.
A gas inlet port 14 is formed through the upper bottom 10a of the reaction
chamber 10, and communicates with a gas supply pump 16 via a gas supply
path 15. In this configuration, a reactive gas, supplied from the gas
supply pump 16, is introduced into the reaction chamber 10 through the gas
supply path 15, dispersed through the gas dispersing holes 13a of the
upper electrode 13 and then led to a plasma generating region 17.
The upper electrode 13 is connected to a first RF power supply 19 via a
first impedance matcher 18. The reactive gas introduced into the plasma
generating region 17 is turned into plasma by the RF power applied by the
first RF power supply 19 to the upper electrode 13.
The sample stage 11 is connected to a second RF power supply 21 via a
second impedance matcher 20. The plasma, generated in the plasma
generating region 17, is made to impinge onto the semiconductor substrate
12 held on the substrate holding portion 11a of the sample stage 11 by the
application of the RF power from the second RF power supply 21 to the
sample stage 11.
A gas outlet port 22 is provided in a sidewall of the reaction chamber 10
and communicates with a gas exhaust pump 23.
Thus, when an etching gas is supplied as a reactive gas from the gas supply
pump 16, plasma is generated from the etching gas in the plasma generating
region 17 and impinges onto the semiconductor substrate 12. As a result, a
film to be processed, which has been formed on the semiconductor substrate
12, is etched with the plasma. Also, redundant plasma and reactants
generated as a result of plasma etching are exhausted through the gas
outlet port 22 to the outside.
The first embodiment is characterized in that the main members of the
reaction chamber 10, namely, upper bottom 10a, lower bottom 10b, sidewall
10c, sample stage 11 (except for the substrate holding portion 11a) and
periphery of the upper electrode 13, are covered with synthetic quartz
protective members 25. And a view port 26 for observing the inside of the
reaction chamber 10 is also made of synthetic quartz.
In the first embodiment, the main members of the reaction chamber 10 are
covered with the protective members 25 made of synthetic quartz, and the
view port 26 is also made of synthetic quartz. Thus, even when the
protective members 25 and the view port 26 are exposed to and etched by
the plasma generated in the plasma generating region 17, substantially no
impurities, such as sodium, calcium, potassium, aluminum, iron or
magnesium, are released from the protective members 25 or the view port
26.
Since substantially no impurities are released from the protective members
25 or the view port 26, substantially no impurities are deposited onto a
film to be processed that is formed on the surface of the semiconductor
substrate 12. Accordingly, the pause time of a DRAM formed on the
principal surface of the semiconductor substrate 12 can be increased to a
large degree.
FIG. 2 illustrates the cross-sectional structure of the principal portion
of a conventional plasma processing apparatus, which corresponds to the
portion A encircled by the one dot chain in FIG. 1. In FIG. 2, the same
members as those of the plasma processing apparatus of the first
embodiment shown in FIG. 1 are identified by the same reference numerals
and the description thereof will be omitted herein. As shown in FIG. 2,
the main members of the reaction chamber of the conventional plasma
processing apparatus are covered with protective members 1 made of fused
quartz.
In the conventional plasma processing apparatus, when the fused-quartz
protective members 1 are exposed to plasma generated in the plasma
generating region 17, a lot of impurities are released from some of the
protective members 1 and deposited onto the surface of the semiconductor
substrate 12 or re-deposited onto other protective members 1. As a result,
the pause time of a DRAM formed on the principal surface of the
semiconductor substrate 12 adversely decreases.
FIG. 3 illustrates the relationship between the content of impurities in
the protective members and the amount of impurities deposited onto the
surface of the semiconductor substrate. As shown in FIG. 3, it can be seen
that the larger the amount of impurities contained in the protective
members is, the larger the amount of impurities deposited onto the
substrate is. In other words, the smaller the amount of impurities
contained in the protective members is, the smaller the amount of
impurities deposited onto the substrate is.
In FIG. 3, a relationship between the content of impurities in the
protective members 1, made of fused quartz conventionally used
(abscissas), and the amount of deposited impurities with the protective
members 1 made of fused quartz used (ordinates) is shown in the upper
right domain. A relationship between the content of impurities in the
protective members 25, made of synthetic quartz in the first embodiment
(abscissas), and the amount of deposited impurities with the synthetic
quartz protective members 25 used (ordinates) is shown in the lower left
domain of FIG. 3. As is clear from FIG. 3, the amount of impurities
deposited onto the surface of the semiconductor substrate can be reduced
to less than 1.times.10.sup.10 in accordance with the first embodiment.
FIG. 4 illustrates the relationship between the amount of impurities
deposited onto the surface of the semiconductor substrate (abscissas) and
the pause time (charge retention time; ordinates). As can be understood
from FIG. 4, the larger the amount of impurities deposited is, the shorter
the pause time is. And if the amount of impurities deposited is
2.times.10.sup.10 or more, the pause time abruptly decreases.
According to the first embodiment, the amount of impurities deposited onto
the surface of the semiconductor substrate can be reduced to less than
1.times.10.sup.10 as described with reference to FIG. 3. Thus, as can be
seen from FIG. 4, the pause time can be greatly reduced.
Hereinafter, calculated and actually measured values of the amount of
impurities deposited onto the surface of a semiconductor substrate will be
described for the cases of using fused-quartz protective members as in the
prior art and using synthetic-quartz protective members as in the first
embodiment.
The calculated value of the amount A of impurities deposited on the surface
of a semiconductor substrate is obtained by:
A=DVCN/(MS)
where D is the density of the protective members, V is the volume of the
protective members, C is the content of impurities in the protective
members, N is Avogadro's number, M is the molecular weight of the
impurities and S is the area of the semiconductor substrate.
On the other hand, the actually measured value of the amount of impurities
deposited on the surface of the semiconductor substrate was obtained as
follows. Specifically, a silicon dioxide film formed over the entire
surface of a silicon substrate was dry-etched on the following conditions:
Etching gas: C.sub.4 F.sub.8 /CO/Ar/O.sub.2 =5-20/5-20/100-400/5-20 sccm
RF power applied to upper electrode: 1,000 to 2,000 W
RF power applied to lower electrode: 500 to 1,500 W
Etching time: 30 to 120 seconds
Then, the surface of the silicon substrate was dissolved with HF vapor and
the solution was recovered into 0.7 wt % HNO.sub.3 solution. Next, the
recovered solution was analyzed by flameless atomic absorption
spectroscopy.
TABLE 1
Na Ca K Al Fe Mg
Impurity Content 0.8-0.4 0.4-1.0 0.2-0.8 9.0-50 0.3-0.7 0.1
(ppm)
Typical Impurity 1.0 1.0 0.7 10.0 0.7 0.1
Content (ppm)
Impurity Deposit- 51.7 29.7 21.3 440 14.9 4.89
ed (calcu-
lated; .times.10.sup.10 cm.sup.2)
Impurity Deposit- 24.0 13.0 3.6 7.8 2.3 1.7
ed (measur-
ed; .times.10.sup.10 cm.sup.2)
Table 1 illustrates a case of using fused quartz. Fused quartz contains the
impurities of: sodium (content: 0.8 to 4.0 ppm); calcium (content: 0.4 to
1.0 ppm); potassium (content: 0.2 to 0.8 ppm); aluminum (content: 9.0 to
50 ppm); iron (content: 0.3 to 0.7 ppm); and magnesium (content: 0.1 ppm).
These data are extracted from the catalogs of Tokyo Fine Glass Corp. and
Asahi Glass Co., Ltd., and Chemistry Handbook (Developed Course) edited by
Japan Chemical Industry Association. Accordingly, the typical values of
the impurities contained in fused quartz are herein defined at 1.0 ppm,
1.0 ppm, 0.7 ppm, 10.0 ppm, 0.7 ppm and 0.1 ppm for sodium, calcium,
potassium, aluminum, iron and magnesium, respectively.
If the contents of impurities are defined at these typical values, then the
amounts of impurities on the semiconductor substrate are calculated as
51.7.times.10.sup.10 /cm.sup.2, 29.7.times.10.sup.10 /cm.sup.2,
21.3.times.10.sup.10 /cm.sup.2, 440.times.10.sup.10 /cm.sup.2,
14.9.times.10.sup.10 /cm.sup.2 and 4.89.times.10.sup.10 /cm.sup.2 for
sodium, calcium, potassium, aluminum, iron and magnesium, respectively.
The actually measured values of the amounts of impurities are
24.0.times.10.sup.10 /cm.sup.2, 13.0.times.10.sup.10 /cm.sup.2,
3.6.times.10.sup.10 /cm.sup.2, 7.8.times.10.sup.10 /cm.sup.2,
2.3.times.10.sup.10 /cm.sup.2 and 1.7.times.10.sup.10 /cm.sup.2 for
sodium, calcium, potassium, aluminum, iron and magnesium, respectively.
TABLE 2
Na Ca K Al Fe Mg
Impurity Content <0.07 <0.1 <0.03 <0.05 <0.1 <0.02
(ppm)
Typical Impurity 0.02 0.02 0.02 0.03 0.02 0.02
Content (ppm)
Impurity Deposited 1.003 0.593 0.608 1.32 0.426 0.979
(calculated; .times.10.sup.10 cm.sup.2)
Impurity Deposited 0.895 0.364 0.237 0.802 0.328 0.194
(measured; .times.10.sup.10 cm.sup.2)
Table 2 illustrates a case of using synthetic quartz. Synthetic quartz
contains the impurities of: sodium (content: to less than 0.07 ppm);
calcium (content: less than 0.1 ppm); potassium (content: less than 0.03
ppm); aluminum (content: less than 0.05 ppm); iron (content: less than 0.1
ppm); and magnesium (content: less than 0.02 ppm). These data are also
extracted from the catalogs of Tokyo Fine Glass Corp. and Asahi Glass Co.,
Ltd., and Chemistry Handbook (Developed Course) edited by Japan Chemical
Industry Association. Accordingly, the typical values of the impurities
contained in synthetic quartz are herein defined at 0.02 ppm, 0.02 ppm,
0.02 ppm, 0.03 ppm, 0.02 ppm and 0.02 ppm for sodium, calcium, potassium,
aluminum, iron and magnesium, respectively.
If the contents of impurities are defined at these typical values, then the
amounts of impurities on the semiconductor substrate are calculated as
1.003.times.10.sup.10 /cm.sup.2, 0.593.times.10.sup.10 /cm.sup.2,
0.608.times.10.sup.10 /cm.sup.2, 1.32.times.10.sup.10 /cm.sup.2,
0.426.times.10.sup.10 /cm.sup.2 and 0.979.times.10.sup.10 /cm.sup.2 for
sodium, calcium, potassium, aluminum, iron and magnesium, respectively.
The actually measured values of the amounts of impurities are
0.895.times.10.sup.10 /cm.sup.2, 0.364.times.10.sup.10 /cm.sup.2,
0.237.times.10.sup.10 /cm.sup.2, 0.802.times.10.sup.10 /cm.sup.2,
0.328.times.10/cm.sup.2 and 0.194.times.10.sup.10 /cm.sup.2 for sodium,
calcium, potassium, aluminum, iron and magnesium, respectively.
EMBODIMENT 2
Hereinafter, a plasma processing apparatus according to the second
embodiment of the present invention will be described with reference to
FIGS. 5, 6(a) and 6(b).
The plasma processing apparatus of the second embodiment is characterized
in that a ring member 30 made of synthetic quartz is disposed around the
substrate placing portion 11a (i.e., the substrate holding portion) on
which the semiconductor substrate 12 is placed as shown in FIG. 5. Since
the other members are the same as those used in the first embodiment, the
illustration and description thereof will be omitted herein. The
protective members covering the main members of the reaction chamber and
the view port may be made of synthetic quartz as in the first embodiment
or fused quartz as in the prior art.
As shown in FIGS. 6(a) and 6(b), the ring member 30 made of synthetic
quartz includes a stepped portion 30a having an inner diameter slightly
larger than the diameter of the semiconductor substrate in the shape of a
wafer. Thus, the periphery of the semiconductor substrate 12 placed in the
substrate placing region 11a of the sample stage 11 is mounted on the
stepped portion 30a of the ring member 30. Accordingly, the substrate
holding portion 11a of the sample stage 11 is not exposed to the plasma
generating region 17 (see FIG. 1).
As can be understood, the ring member 30 made of synthetic quartz is
disposed around the substrate placing region 11a of the sample stage 11 in
this second embodiment. Thus, even when the ring member 30 is exposed to
and etched by the plasma generated in the plasma generating region 17,
substantially no impurities such as sodium, calcium, potassium, aluminum,
iron and magnesium are released from the ring member 30.
Hereinafter, a plasma etching method using the plasma processing apparatus
of the second embodiment will be described with reference to FIGS. 7(a)
through 7(d).
First, as shown in FIG. 7(a), a silicon dioxide film 32, made of BPSG, is
deposited on a semiconductor substrate 31. Then, as shown in FIG. 7(b), a
resist pattern 33 is formed on the silicon dioxide film 32.
Next, the semiconductor substrate 31, with the resist pattern 33 formed on
the silicon dioxide film 32, is placed on the substrate placing region 11a
of the sample stage 11 in the plasma processing apparatus of the second
embodiment. And the ring member 30 made of synthetic quartz is disposed
around the semiconductor substrate 31.
Subsequently, as shown in FIG. 7(c), the silicon dioxide film 32 is etched
with plasma using the resist pattern 33 as a mask, thereby forming contact
holes 32a in the silicon dioxide film 32. And as shown in FIG. 7(d), the
resist pattern 33 is removed.
In accordance with the plasma etching method performed using the plasma
processing apparatus of the second embodiment, impurities are much less
likely to be released from the ring member 30 made of synthetic quartz
during the process step of etching the silicon dioxide film 32. Thus,
since substantially no impurities are deposited on the surface of the
silicon dioxide film 32, the pause time of a DRAM formed on the principal
surface of the semiconductor substrate 31 can be considerably increased.
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